BACKPORT: cgroup: misc changes

Misc trivial changes to prepare for future changes.  No functional
difference.

* Expose cgroup_get(), cgroup_tryget() and cgroup_parent().

* Implement task_dfl_cgroup() which dereferences css_set->dfl_cgrp.

* Rename cgroup_stats_show() to cgroup_stat_show() for consistency
  with the file name.

Signed-off-by: Tejun Heo <tj@kernel.org>

(cherry picked from commit 3e48930cc74f0c212ee1838f89ad0ca7fcf2fea1)

Conflicts:
        kernel/cgroup/cgroup.c

(1. manual merge because kernel/cgroup/cgroup.c is under kernel/cgroup.c
2. cgroup_stats_show change is skipped because the function dos not exist)

Bug: 111308141
Test: modified lmkd to use PSI and tested using lmkd_unit_test

Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I756ee3dcf0d0f3da69cd1b58e644271625053538